When approaching Earth, asteroids tend to do one of two things: Most of the time, they miss. Sometimes, they hit, leaving a bright streak in our planet’s sky or a gruesome new wound in its crust. But very rarely, asteroids are captured by Earth’s gravity and pirouette around the planet, becoming, in effect, a moon — albeit an ephemeral one.
A space rock spotted recently with NASA-funded ground telescopes is about to do just that, temporarily becoming an additional lunar-like companion to planet Earth.
The asteroid 2024 PT5, reported this month in the Research Notes of the American Astronomical Society, is just 33 feet long. Astronomers calculate that from Sept. 29 to Nov. 25, it will loop around the planet before breaking free of its gravitational tether and flying off into space.
“It is pretty cool,” said Federica Spoto, an asteroid dynamics researcher at the Center for Astrophysics, Harvard & Smithsonian, who was not involved with the study. She added that observations of 2024 PT5 will bolster scientists’ knowledge of the sort of space rocks that flit about close to Earth — including those that occasionally crash into it.
Asteroids that fail to escape Earth’s gravity and end up orbiting the planet for a time are referred to as mini-moons. Being so diminutive and speedy, they are difficult to spot and formally identify. Sometimes, they turn out to be artificial objects: the European Space Agency’s star-mapping Gaia spacecraft was once mistaken for an asteroid. Leftover parts of rockets have also taken on such alter egos.
“Every time an object with an orbit so earthlike is discovered, there is a chance that we are just recovering space debris,” said Raúl de la Fuente Marcos, an astronomer at the Complutense University of Madrid and a co-author of the study. But, he said, observations of 2024 PT5 indicate that “it is a natural object, no doubt about that.”
The asteroid was discovered on Aug. 7 using the NASA-funded Asteroid Terrestrial-impact Last Alert System, or ATLAS. And according to astronomers’ projections, the asteroid is about to perform a two-month-long slingshot around the planet.
The potential origin story of 2024 PT5 offers a quirky plot twist. The asteroid’s past motion suggests that it’s “possibly a piece of ejecta from an impact on the moon,” said Paul Chodas, the director of the Center for Near Earth Object Studies at NASA’s Jet Propulsion Laboratory. In other words, Earth’s new mini-moon could be a miniature fragment of the actual moon.
Despite its possible lunar ancestry, the object may not technically count as a mini-moon.
To normally qualify, an asteroid must orbit Earth fully at least once; 2024 PT5 will perform a horseshoe-shaped orbit. “It certainly won’t complete one full revolution in the Earth-moon system this fall, so I’m not sure I would classify it as a mini-moon’” said Lance Benner, the principal investigator of the asteroid radar research program at the Jet Propulsion Laboratory.
Whether bona fide mini-moons or not, Earth-orbiting asteroids like 2024 PT5 aren’t mere curiosities. Many contain precious metals that, someday, companies hope to extract.
“Every time they’re talking about asteroid mining, they talk about mini-moons,” Dr. Spoto said. A metal-rich space rock that ends up orbiting Earth — perhaps with the aid of a future spacecraft nudging it into position — would be an ideal target for these futuristic prospectors.
Planetary defense researchers are also intrigued by objects like 2024 PT5. They are primarily focused on finding near-Earth objects 460 feet across — those capable of annihilating a city. To date, about 11,000 such asteroids out of a projected total of 25,000 have been identified. But there are millions of smaller, still threatening near-Earth rocks whose whereabouts remain unknown, the sort that could still cause widespread damage and casualties if they impacted a populated area.
The discovery of 2024 PT5 serves as a reminder that “there’s a pretty busy highway around the Earth,” Dr. Spoto said. Knowing where all of the traffic is going, and what it’s like, is of paramount importance to all seven billion of us.
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